Problem: Christopher is 20 years old and Nadia is 2 years old. How many years will it take until Christopher is only 3 times as old as Nadia?
Answer: We can use the given information to write down an equation about how many years it will take. Let $y$ be the number of years that it will take. In $y$ years, Christopher will be $20 + y$ years old and Nadia will be $2 + y$ years old. At that time, Christopher will be 3 times as old as Nadia. Writing this information as an equation, we get: $20 + y = 3 (2 + y)$ Simplifying the right side of this equation, we get: $20 + y = 6 + 3 y$ Solving for $y$ , we get: $2 y = 14$ $y = 7$.